1. Field of Invention
The current invention relates to observation systems and methods of imaging light-sensitive tissue, and more particularly to observation systems and methods of imaging light-sensitive tissue with reduced phototoxicity.
2. Discussion of Related Art
Retinal microsurgery is one of the most demanding types of surgery. The difficulty stems from the microscopic dimensions of tissue planes and blood vessels in the eye, the delicate nature of the neurosensory retina and the poor recovery of retinal function after injury. Many micron-scale maneuvers are physically not possible for many retinal surgeons due to inability to visualize the tissue planes, tremor, or insufficient dexterity. To safely perform these maneuvers, microscopes are required to view the retina. A central issue for the surgeon is the compromise between adequate illumination of retinal structures, and the risk of iatrogenic phototoxicity either from the operating microscope or endoilluminators, which are fiber-optic light sources that are placed into the vitreous cavity to provide adequate illumination of the retina during delicate maneuvers.
Retinal phototoxicity from an operating microscope was first reported in 1983 in patients who had undergone cataract surgery with intraocular lens implantation (McDonald, H., Irvine, A.: Light-induced maculopathy from the operating micro-scope in extracapsular cataract extraction and intraocular lens implantation. Ophthalmology 90, 945-951 (1983)). Retinal phototoxicity is now a well recognized potential complication of any intraocular surgical procedure, and the frequency is reported to occur from 7% to 28% of patients undergoing cataract surgery (Khwarg, S., Linstone, F., Daniels, S., Isenberg, S., Hanscom, T., Geoghegan, M., Straatsma, B.: Incidence, risk factors, and morphology in operating microscope light retinopathy. American Journal of Ophthalmology 103, 255-263 (1987); Byrnes, G., Antoszyk, A., Mazur, D., Kao, T., Miller, S.: Photic maculopathy after extracapsular cataract surgery a prospective study. Ophthalmology 99, 731-738 (1992)). As a result, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) now provides safety guidelines for illumination of the fundus in both phakic and aphakic subjects (International Commission on Non-Ionizing Radiation Protection: Guidelines on limits of exposure to broad-band incoherent optical radiation (0.38 to 3). Health Phys. 73, 539-554 (1997)). Blue wavelength and ultraviolet light induce the greatest degree of retinal injury. In fact, in (van den Biesen, R., Berenschot, T., Verdaasdonk, R., van Weelden, H., van Norren, D.: Endoillumination during vitrectomy and phototoxicity thresholds. British Journal of Ophthalmology 84, 1372-1375 (2000)) it was found that commercially available light sources for endoillumination exceeded the ICNIRP guidelines for retinal damage by visible light within 3 minutes, and in 9 of 10 sources, the safe exposure time was exceeded in less than 1 minute. In vitrectomy for macular hole repair, up to 7% of the patients have been reported to have experienced visually significant phototoxicity (Poliner, L., Tornambe, P.: Retinal pigment epitheliopathy after macular hole surgery. Ophthalmology 99, 1671-1677 (1992); Michels, M., Lewis, H., Abrams, G., Han, D., Mieler, W., Neitz, J.: Macular pho-totoxicity caused by fiberoptic endoillumination during pars plana vitrectomy. American Journal of Ophthalmol. 114, 287-292 (1992); Banker, A., Freeman, W., Kim, J., Munguia, D., Azen, S.: Vision-threatening complications of surgery for full-thickness macular holes. Ophthalmology 104, 1442-1453 (1997)).
Phototoxicity can be either thermal or photochemical in nature from excessive ultraviolet (UV) or blue light toxicity. Ham et al. showed the action spectrum or relative risk of UV or blue light toxicity when the retina was exposed to various wavelengths of light (Ham, W. J., Mueller, H., Ru olo, J. J., Guerry, D., Guerry, R.: Action spectrum for retinal injury from near-ultraviolet radiation in the aphakic monkey. American Journal of Ophthalmology 93, 299-306 (1982)). The action spectrum was then used to create a relative risk of phototoxicity associated with a given wavelength of light.
The Aphakic Hazard Function describes the phototoxic potential of retinal light exposure within and near the visible spectrum. As seen from the curve in FIG. 1, retinal phototoxicity occurs primarily at short wavelengths, such as blue light. Red light has little to no phototoxic impact compared to blue light.
Current medical light sources attempt to limit phototoxicity by using filters to block wavelengths at the blue end of the visible spectrum. This approach has only limited usefulness, however, since blocking part of the visible spectrum hinders color rendition. Xenon is currently the illumination source of choice for retinal surgery. As shown on the Aphakic Hazard Function diagram (FIG. 1)(Ohji, Masahito and Tan θ, Yasuo. Vitreo-retinal Surgery. “Chapter 7: New Instruments in Vitrectomy”. Berlin Heidelberg: Springer, 2007), a Xenon spectrum is fairly flat and has substantial coverage within the hazardous blue wavelength range. Thus, the industry standard light source for retinal surgery may be a significant health hazard for patients. The risk, while reduced, is still significant for intraocular surgery. Given the advancing age of the population and increasing prevalence of retinal diseases, there remains a need for further improvements aimed at reducing iatrogenic retinal phototoxicity.